Method and structure to reduce optical crosstalk in a solid state imager

ABSTRACT

Methods and structures to reduce optical crosstalk in solid state imager arrays. Sections of pixel material layers that previously would have been etched away and disposed of as waste during fabrication are left as conserved sections. These conserved sections are used to amend the properties and performance of the imager array. In the resulting structure, the conserved sections absorb incident light. The patterned portions of conserved material provide additional light shielding for array pixels.

FIELD OF THE INVENTION

This invention generally relates to electronic systems and in particularit relates to semiconductor image sensing devices.

BACKGROUND OF THE INVENTION

Optical crosstalk may exist between neighboring photosensors in a pixelarray of a solid state imager, such as a CCD or CMOS imager, forexample. Optical crosstalk in imagers can bring about undesirableresults in images that they produce. The undesirable results can becomemore pronounced as the density of pixels in imager arrays increases, andas pixel size correspondingly decreases.

In an idealized photosensor, a photodiode for example, light enters onlythrough the surface of the photodiode that directly receives the lightstimulus. In reality, however, light intended for neighboringphotosensors also enters the photodiode, in the form of stray light,through the sides of the photosensor structure for example. Reflectionand refraction within the photosensor structure can give rise to thestray light, which is referred to as “optical crosstalk.”

Optical crosstalk can manifest as blurring or reduction in contrast, forexample, in images produced by a solid state imager. As noted above,image degradation can become more pronounced as pixel and device sizesare reduced. Degradation caused by optical crosstalk also is moreconspicuous at longer wavelengths of light. Light at longer wavelengthspenetrates more deeply into the silicone structure of a pixel, providingmore opportunities for the light to be reflected or refracted away fromits intended photosensor target.

Problems associated with optical-crosstalk have been addressed by addinglight shields to imager structures. The light shields are formed inlayers fabricated above the admitting surface through which thephotosensor directly receives light stimulus. The light shield layersgenerally include metal and other opaque materials. The added layers oflight shields, however, increase the size, complexity, and cost ofimagers and imager fabrication.

The added light shields generally are formed as part of the uppermostlayers of the imager array. Light shields have been formed, for example,in metal interconnect layers (e.g., Metal 1, Metal 2, or, if utilized,Metal 3 layers) of the photosensor's integrated circuitry. Light shieldsformed in such upper fabrication layers have inherent drawbacks,however. For example, metallization layers dedicated to light shieldingare limited in their normal use as conductive connections for the imagerarray. Additionally, light shields formed in upper device layers areseparated from the light-admitting surface of the photosensor by severallight transmitting layers. Moreover, the light shields are imperfect,and allow some light to pass into the light transmitting layers.Consequently, optical crosstalk still occurs through the lighttransmitting layers between the photosensor and the light shields.Having the light shields spaced apart from the surface of thephotosensor also can increase light piping and light shadowing in thephotosensors, leading to further errors in imager function.

Methods and structures related to light shielding in CMOS imagers aredisclosed in U.S. Pat. No. 6,611,013 to Rhodes, and U.S. Pat. No.6,333,205, also to Rhodes. The Rhodes '013 and '205 patents areincorporated herein by reference in their entirety.

Solid state imagers would benefit from more efficient and effectivelight shields. Of particular benefit would be light shields that makemore efficient use of existing fabrication layers to better precludeoptical crosstalk.

BRIEF SUMMARY OF THE INVENTION

The present invention in various exemplary embodiments providesfabrication methods and resulting pixel array structures in whichindividual fabrication layers normally discarded in pixel fabricationare patterned into sections that can serve light shield or collectionpurposes.

In one exemplary embodiment, a fabrication layer includes polysiliconpatterned to develop transistor gate structures as part of a pixelcircuit in a first section (referred to as a “circuit section”).Structures designed to physically augment light shielding or lightcollecting capabilities of the pixel are formed in a second section(referred to as a “conserved section”) of the fabrication layer. Theconserved section previously was etched away during imager fabricationas unusable waste. According to exemplary embodiments of the presentinvention, the conserved section instead is retained and used to augmentthe pixel structure and function. For example, the conserved section canbe used to enhance light shielding or for light collection.

According to an embodiment exemplary of the invention, at least onepolysilicon layer is patterned into a pixel gate or pixel interconnectcircuit section and a conserved section. The gate circuit section isdeveloped for discrete transistor gate fabrication while theinterconnect circuit section may be used to interconnect electricalelements of the pixel. The conserved section is left undisturbed,instead of being etched away, and is used as a light shield or for lightcollection.

Advantageously, if the conserved section of the polysilicon layer usedfor light shielding, it is located adjacent an energy-admitting face ofthe photosensor. The close proximity of the light shields formed by theconserved sections of the polysilicon layer prevent scattering of lightto neighboring pixels. The light shields thus formed make economical andvaluable use of material previously considered waste. The conservedsection, of the polysilicon layer for example, may also be used forlight collection purposes. The conserved sections of an existing layercan be used to replace or augment the functions of other layers, whichother layers accordingly can be removed or made thinner, resulting in amore compact pixel construction.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other advantages and features of the invention will bemore dearly understood from the following detailed description which isprovided in connection with the accompanying drawings.

FIG. 1 illustrates in elevation a cross section, taken along the lineI-I in FIG. 3, of a portion of an imager array according to an exemplaryembodiment of the present invention;

FIG. 2 is a circuit diagram of a pixel in the imager array of FIG. 1;

FIG. 3 is a plan view showing pixels arranged in a portion of a row andcolumn imager array in an intermediate state of fabrication according tothe exemplary embodiment;

FIG. 4 illustrates a cut away side view of a pixel-containing portion ofa semiconductor CMOS imager wafer in an initial stage of processingaccording to an exemplary embodiment of the invention;

FIG. 5 illustrates a cut away side view of a portion of the exemplarysemiconductor CMOS imager wafer at a processing stage subsequent to FIG.4;

FIG. 6 illustrates a cut away side view of a portion of the exemplarysemiconductor CMOS imager wafer at a processing stage subsequent to FIG.5;

FIG. 7 illustrates a cut away side view of a portion of the exemplarysemiconductor CMOS imager wafer in an interim stage of processingsubsequent to FIG. 6;

FIG. 8 illustrates a cut away side view of a portion of the exemplarysemiconductor CMOS imager wafer at a processing stage subsequent to FIG.7;

FIG. 9 illustrates a cut away side view of a portion of the exemplarysemiconductor CMOS imager wafer at a processing stage subsequent to FIG.8;

FIG. 10 is a partially cut away side view of a portion of the exemplarysemiconductor CMOS imager wafer at a processing stage subsequent to FIG.9;

FIG. 11 illustrates a partially cut away side view of a portion of theexemplary semiconductor CMOS imager wafer at a processing stagesubsequent to FIG. 10;

FIG. 12 is a block diagram of an exemplary imaging device including anarray of imager pixels as illustrated in FIGS. 1-11;

FIG. 13 illustrates a processing system including a CMOS imageraccording to an exemplary embodiment of the present invention; and

FIG. 14 illustrates another exemplary embodiment of the invention inwhich sections of a layer used to form a top cell plate of a capacitorare conserved for light shielding or collecting purposes.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof and illustrate specificexemplary embodiments by which the invention may be practiced. It shouldbe understood that like reference numerals represent like elementsthroughout the drawings. These embodiments are described in sufficientdetail to enable those skilled in the art to practice the invention. Itis to be understood that other embodiments may be utilized, and thatstructural, logical, and electrical changes may be made withoutdeparting from the spirit and scope of the present invention.

The term “substrate” is to be understood as includingsilicon-on-insulator (SOI) or silicon-on-sapphire (SOS) technology,doped and undoped semiconductors, epitaxial layers of silicon supportedby a base semiconductor foundation, and other semiconductor structures.Furthermore, when reference is made to a “substrate” in the followingdescription, previous process steps may have been utilized to formregions or junctions in the base semiconductor structure or foundation.In addition, the semiconductor need not be silicon-based, but could bebased on silicon-germanium, germanium, or gallium arsenide, for example.

The term “light” refers to electromagnetic radiation that can produce avisual sensation (visible light) as well as electromagnetic radiationoutside of the visible spectrum. In general, light as used herein is notlimited to visible radiation, but refers more broadly to the entireelectromagnetic spectrum, particularly electromagnetic radiation thatcan be transduced by a solid state photosensor into a useful electricalsignal.

The term “pixel” refers to a picture element unit containing circuitryincluding a photosensor and transistors for converting incidentelectromagnetic radiation to an electrical signal. For purposes ofillustration, representative pixels are illustrated in the drawings anddescription herein. Typically fabrication of all pixels in an imagerwill proceed simultaneously in a similar fashion. The following detaileddescription is, therefore, not to be taken in a limiting sense, and thescope of the present invention is defined by the appended claims.

The term “layer” refers to both a single layer and multiple layers, orstrata. The term ‘layer’ can be understood to refer to a structure thatincludes multiple layers. Typically, similar fabrication steps andprocesses, such as patterning and etching, are applied to all layers inthe structure. Adjacent layers can be patterned and etchedsimultaneously.

The present invention provides an imaging device formed in an exemplaryembodiment as a CMOS integrated circuit using standard CMOS fabricationprocesses. The exemplary embodiment provides fabrication methods andresulting pixel array structures in which individual fabrication layersare patterned into sections serving assorted purposes. For example, apolysilicon layer traditionally is patterned to develop transistor gatestructures for pixel circuitry. Sections of the polysilicon layer notused as transistor gate structures, formerly discarded as unusable, areinstead conserved for light shielding or light collecting, for example.Sections of other fabrication layers, besides transistor gatepolysilicon layers, can be similarly conserved and used for lightshielding. A gate oxide layer formed beneath and coextensively with thepolysilicon layer can be patterned simultaneously with the polysiliconlayer to provide additional light shielding or collecting capabilities.

In some pixel architectures the polysilicon layer may also be used as aburied contact to reduce the vertical distance a metal contact musttravel. In such arrangements, the conserved sections include areas ofthe polysilicon layer not used for gates or interconnect structures.These conserved sections can be used as light shields or lightcollectors.

In some pixel architectures the polysilicon layer or metallic layers mayalso be used as a local interconnect to interconnect pixel components.In such arrangements, the conserved sections include areas of thepolysilicon layer and metallic layers not used for gates or interconnectstructures. These conserved sections can be used as light shields orlight collectors.

In yet other pixel architectures an upper polysilicon layer, above thepolysilicon gate layer, may be used to form upper electrodes of acapacitor for a pixel. In such arrangements, the upper polysilicon layermay also be patterned to have conserved sections which are not used forthe capacitor electrode and may act as light shields or for collectinglight.

In the exemplary embodiments described below, the invention is discussedin the context of patterning a polysilicon layer and associated oxidelayer into a gate circuit section for the pixel and into a conservedsection which is not used for gate or interconnected functions and whichcan be used as a light shield or for light gathering functions. However,it should be understood that other fabrication layers close to thesubstrate surface, such as the capacitor electrode polysilicon layerdiscussed above or any polysilicon layers used for buried contacts orlocal interconnects, may also be patterned to yield conserved sectionswhich can be used for light shielding or light gathering purposes.

Referring now to the polysilicon layer used for pixel gates, thepolysilicon gate layer and the underlying gate oxide layer are providedover substantially the full extent of the substrate's major surface.Openings are provided in the polysilicon gate layer and the gate oxidelayer to allow light to impinge upon an admitting face of thephotosensor. The openings are formed typically by patterning and etchingaway layers formed over those sections required for physical developmentand electrical isolation of transistor gates and other devicecomponents, as described below. The polysilicon layer and thecoextensive gate oxide layer are configured to closely border outermargins of pixel active areas to provide protection fromphotocontamination by off-axis light from neighboring pixels, forexample, and thereby reduce optical crosstalk.

As a result of using the sections of the polysilicon gate layer toprovide layer light shielding at a level closer to the admitting surfaceof the photocollector, other light-shielding structures, such as thoseprovided in the upper fabrication layers, can be reduced in thickness,for example, or eliminated entirely. The conserved sections of thepolysilicon gate layer, augmented by the gate oxide layer, mayindependently provide sufficient light shielding, in which caselight-shielding structures provided in the upper layers above thephotodiode surface are not necessary. The unnecessary layers can beeliminated or put to another purpose, such as circuit interconnection.As noted, the conserved sections may also be used for light gatheringfunctions.

If sufficient light shielding is not provided by the retained conservedsections of the polysilicon gate layer and the gate oxide layer, forexample, additional light shielding in upper device layers can continueto be employed. As a result of the added light-shielding capabilitiesprovided by the lower layer light shields, however, the thickness and/orextent of the upper light-shielding layers can be reduced whileretaining or improving upon the overall level of light shielding in thepixel. By reducing reliance on upper layer light-shields, and by usingconserved sections of an existing layer that previously were discardedas having no useful function, fabrication processes can be simplified,and device sizes can be further reduced.

Further economies in fabrication can be realized by retaining theconserved sections of the polysilicon gate layer, for example. Leavingthe conserved sections in place presents a more planar surface forsubsequent deposition steps, as compared to a surface with the conservedsections of polysilicon removed. As a result, layers subsequentlydeposited can be smoother, even without polishing the polysilicon gatelayer. Steps previously required to polish the polysilicon gate layerprior to subsequent deposition can be eliminated or reduced in duration.Concomitantly, the layers deposited over the polysilicon gate layer willbe smoother, thereby eliminating or reducing the need for subsequentpolishing steps.

Exemplary embodiments of the methods and apparatus aspects of theinvention are described below in connection with CMOS imager circuitryand components. The circuit described below includes a photosensorformed as a photodiode, for accumulating photo-generated charge in anunderlying portion of the substrate. It should be understood, however,that the imager may include a photogate, or any other image-to-chargeconverter or transducer, in lieu of a photodiode. The invention is notlimited by the type of device used to accumulate or otherwise obtainphoto-generated charge. It should also be noted that while an embodimentof the invention is described in relation to four-transistor (4T) pixelsof a CMOS imager, the invention also has applicability to other pixelstructures and to other types of solid state imagers that feature pixelarrays.

Referring more specifically to the drawings, FIG. 1 shows incross-section a portion of CMOS pixel 12 and portions of adjacent pixels11, 13 exemplifying an embodiment of the present invention. A schematiccircuit representation of the pixel 12 is provided in FIG. 2. Thecircuit is shown as a four transistor (4T) pixel, though, as noted, thepixel may have any of several circuit configurations. Portions of theFIG. 2 circuit appear in the FIG. 1 cross section, including photodiode14, transfer gate 24 (of transfer transistor 25), reset gate 28 (ofreset transistor 30), and a floating diffusion node 26.

FIG. 3 illustrates in plan view a section of the CMOS imager pixel arrayshown in FIGS. 1 and 2. FIG. 3 depicts the pixels 12, 13 and portions ofneighboring pixels in the imager pixel array. It is understood thatadditional pixels not shown are arranged in row and column fashion tomake up the entire imager pixel array.

The pixels are represented in FIG. 3 at an intermediate stage offabrication according to an exemplary embodiment of the presentinvention. At this point of the fabrication process, described ingreater detail below, a gate oxide layer 46 and a polysilicon layer 48(shown in FIG. 1) have been patterned to provide light access to theadmitting surface of photodiodes 14. The gate oxide layer 46 and thepolysilicon layer 48 are patterned to form, for each photodiode 14, atransfer transistor gate 24, a reset transistor gate 28, a sourcefollower gate 32 and a row select gate 34. (As with other structures,the gates for only one photodiode 14 are numbered to avoid crowding.)The patterning leaves in place conserved sections 54 of the gate oxidelayer 46 and the polysilicon layer 48 for use as light shields, or forlight gathering, likewise described in greater detail below.

Referring in greater detail to FIGS. 1-3, representative pixel 12 of theexemplary imager array includes the photodiode 14 formed by implantationin an epitaxial (EPI) p-type layer 16, which is over a p-type substrate17. An n-type conductivity region 18 is provided in EPI layer 16 andaccumulates photo-generated charge. An uppermost, thin p-typeconductivity region 20 is provided over the n-type region 18. The pixel12 further includes a doped p-well 22 defined in p-type layer 16. Anidentical p-well 23 is provided in layer 16 as part of pixel 13. Above aportion of p-well 22 and adjacent the photodiode 14 the transfer gate 24is formed. The transfer gate 24 serves as part of the transfertransistor 25 (FIG. 2) for electrically gating charges accumulated byphotodiode 14 to floating diffusion region 26 implanted in a portion ofp-well 22.

The reset gate 28 is formed as part of the reset transistor 30 besidethe transfer gate 24, illustrated schematically in FIG. 2. The resettransistor 30 is connected to a voltage source (V_(dd)) through asource/drain region having a conductor 31 (FIG. 2) providing a resettingvoltage to the floating diffusion region 26. Also shown in FIG. 2 arethe source follower gate 32 of source-follower transistor 33, and therow select gate 34 of row select transistor 35. The source follower gate32 and the row select gate 34 are also depicted in FIG. 3.

A conductor 36 at the floating diffusion region 26 is in electricalcommunication with the source follower gate 32 of the source followertransistor 33 through another conductor 38 (FIG. 2). Conductor 38 routesthrough a conductive path in an interconnect layer 40 (e.g., the M1layer). Lateral isolation between the adjacent pixels 11, 12 and 12, 13is provided by shallow trench isolation (STI) regions 42, 44,respectively, illustrated in FIG. 1.

The gate oxide layer 46 and the polysilicon layer 48, introduced abovewith reference to FIG. 3, are formed on or near the upper surface of thelayer 16. In an exemplary embodiment, the gate oxide layer 46 isdeposited over the entire upper surface of the layer 16, followed by thepolysilicon layer 48. The polysilicon layer 48 can be undoped, doped insitu, or subsequently implanted with a dopant, for example. Aninsulative capping layer 50 (made of, e.g., tetraethyl orthosilicate(TEOS), Si(OC₂H₅)₄, oxide, or nitride) is fabricated over thepolysilicon layer 48. Formation of the insulative capping layer 50 canoptionally be preceded in another exemplary embodiment by a silicidelayer 52. These layers 46, 48, 50, (optional 52), are then masked with apatterned photoresist for etching.

Etching of the masked layers 46, 48, 50, (52) selectively removesunmasked portions and leaves in place a circuit section 19 (shown withright leaning cross hatching) of the masked layers including transfergate 24, transfer transistor 25, reset gate 28, reset transistor 30,source-follower gate 32, source-follower transistor 33, row select gate34, and row select transistor 35, respectively. Conserved sections 54(shown in FIG. 3 with left leaning cross hatching) are masked and remainin place as well after etching. In the exemplary embodiment beingdescribed, the conserved sections 54 can be left passive andelectrically isolated from all other circuitry by removal of additionalunmasked sections of the layers 46, 48, 50, (52). Unmasked sectionsimmediately above the admitting face of photodiode 14 also are removedto allow light to enter.

Each conserved section 54 is shown in FIG. 3 as covering a contiguousarea, but the invention is not so limited. Additional areas, which mayor may not be contiguous with conserved section 54, can be consideredpart of the conserved section 54 and act as light shields in similarfashion, taking into consideration manufacturing tolerances andrequirements, and potential component and structural interactions, e.g.,capacitance, inductance, etc.

Referring back to FIG. 1, the layers 46, 48, 50, (52), and others, areshown as being substantially planar. Further, although only optionalsilicide layer 52 has been indicated as part of an exemplary alternativeconstruction, other layers can be added, such as nitride layers, orremoved, by methods that provide devices included within the scope ofthe invention. In addition, it will be understood that FIG. 3illustrates but one exemplary layout of an imager array in which thetransfer gates are arranged on the page vertically with the arraycolumns; however, other pixel layouts may also be used. As but oneexample, the imager array could include a 90°-rotated orientation, inwhich the transfer gates are arranged horizontally with the array rows,and a single, continuous transfer gate for each row could be used.

Referring once again to FIG. 3, the photoactive area of photodiode 14has an outer margin or border 56. Shielding can be maximized by formingthe polysilicon layer and underlying axial layer, for example, as closeas possible to this border. Some light shielding is inherently suppliedby the circuit section 19 containing portions of the transfer transistorgate 24, the reset transistor gate 28, and the row select gate 34 formedwithin the layers 46, 48, 50, (52). Considering photodiodes 14 as beingin columns oriented vertically on the page containing FIG. 3, theassociated circuit sections 19 are patterned to border adjacent theouter left side and right side margins 56 of the photodiodes 14. Thecircuit sections 19 thus provide light shielding to neighboringphotodiodes 14. Thus, areas between photodiodes 14 and between otherpixel pairs in each column of the imager array are provided with lightshielding by the pixel circuitry.

Areas between adjacent photodiodes 14 in each row of photodiodes andcircuit sections 19 of the array, as illustrated in FIG. 3, contain veryfew gates or other structures. As exemplified, these areas accordinglyare covered by conserved sections 54 of layers 46, 48, 50, (52). Inaddition, conserved section 54 covers areas between adjacent gate areasof each pixel 12 left open by the circuit sections 19. Thus, circuitsections 19 and conserved sections 54 cover virtually all of thenon-photoactive areas of each pixel 12.

In the exemplary embodiment shown in FIG. 3, the active area ofphotodiode 14 is substantially quadrangular, and layers 46, 48, 50, (52)bound on all four edges at least most portions of the outer border 56 ofphotodiode 14. The circuit sections 19 and conserved sections 54combined provide light shielding over substantially all of each pixel'ssurface area not occupied by photodiode 14. Margin area gaps between theouter border 56 of photodiode 14 and the adjoining edges of layers 46,48, 50, (52) are minimized within manufacturing tolerances to reduce theamount of stray light that can pass through the gaps and cause crosstalk, while ensuring that shorting of active areas by polysilicon layer48 does not reduce yield unacceptably. Portions of polysilicon layer 48are removed from narrow areas between and adjacent at least some of thegates 24, 28, 32, 34 to prevent problems such as those related toshorting, for example.

Light shielding provided by the conserved sections 54 is due primarilyto light absorption in polysilicon layer 48. The energy bandgap (λ_(G))of silicon is 1.11 eV at 300° K, or a wavelength (λ_(G)) of 1117.8 nm.Photons with a wavelength λ less than λ_(G) are absorbed by theelectrons in the polysilicon lattice. Red light (λ=600-750 nm)penetrates the deepest before becoming absorbed. Green light (λ=500-600nm) penetrates less, while blue light (λ=400-500 nm) is quicklyabsorbed. Polysilicon will absorb about five times more light thancrystal silicon due to the surface roughness and higher absorption.Additional light shielding is exhibited by absorption in other layers ofconserved sections 54, such as the optional silicide layer 52.

More generally, absorption is defined as the relative decrease ofirradiance Φ per unit path length:

δΦ(x)/Φ=αδs  Eq. 1

A solution to this equation is:

Φ(x)=Φ_(o) e ^(−ax)  Eq. 2

where Φ_(o) is the incident irradiance, α is the absorption coefficient,and x is path length.

The absorption coefficient of polysilicon was determined experimentallyby Lubberts et al., “Optical Properties of Phosphorus-dopedPolycrystalline Silicon Layers,” J. Appl. Phys. 52, 6870-6878 (November1981), results of which are shown in the following Table:

TABLE α undoped Wavelength (μm) (×10E4 cm⁻¹) 0.4 22.7 0.45 8.33 0.5 3.70.55 1.84 0.6 0.981

The results shown in the Table reveal that light of longer wavelengths(e.g., red colored light) will be absorbed less (i.e., have a lowerabsorption coefficient a) than light of longer wavelength (e.g., greencolored and blue colored light) for layers of a given thickness. Inaddition, thicker layers absorb more light, with red, green, and bluebeing absorbed at different rates. Layer thickness can be used tocontrol how much light of a certain color is filtered.

In an exemplary embodiment, polysilicon layer 48 has a thickness ofabout 850A, but can range from about 200A to about 3000A, for example.Most of the light shielding in the exemplary embodiments described aboveis provided by light absorption in the polysilicon layer 48. Additionalabsorption takes place in the gate oxide layer 46 and insulative cappinglayer 50. Light shielding also occurs at layer interfaces, e.g.,interfaces between layer pairs 46, 48 and 48, 50.

Referring again to FIG. 2, the representative pixel 12 is operated as isknown in the art by RESET, TRANSFER, and ROW SELECT control signals. Asan example of an alternative exemplary circuit configuration, the 4Tpixel 12 can be converted to a three transistor (3T) pixel by removal ofthe transfer transistor 25, and electrically coupling the photodiode 14output to the floating diffusion region 26. Also, the floating diffusionregion 26 is connected to the source follower gate 32 of the sourcefollower transistor 33 in the 3T pixel circuitry.

FIGS. 4-11 illustrate more completely one exemplary pixel fabricationmethod for an exemplary imager featuring 4T pixel 12. Referringinitially to FIG. 4, a preliminary stage of fabrication involvesdevelopment of enhancement regions in the p-type EPI layer 16 providedover a p-type substrate 17. The trench isolation regions 42, forexample, are formed within the layer 16 and surround active regions ofpixel 12. The trench isolation regions 14 preferably are shallow trenchisolation (STI) regions, but may also be formed without limitation bylocal oxidation of silicon (LOCOS) processing, for example. The trenchisolation regions 42 are formed using a photoresist mask, patterning,and etching to leave trench isolation regions 42 where desired. Thephotoresist is removed, and a layer of dielectric material (e.g.,silicon dioxide, silicon nitride, oxide-nitride, nitride-oxide,oxide-nitride-oxide, etc.) is formed within the trenches by CVD(chemical vapor deposition), LPCVD (low pressure CVD), HDP (high-densityplasma), or other suitable means. After filling the trenches with thedielectric material, the wafer is planarized by chemical-mechanicalpolishing (CMP) or reactive ion etch (RIE) dry etching processes, forexample.

Referring t6 FIG. 5, the transfer gate 24, the reset gate 28, the sourcefollower gate 32, and the row select gate 34 are formed. (Only gates 24and 28 are shown in FIG. 5.) Standard MOS gates typically are providedby forming a gate oxide layer 46 (e.g., silicon oxide) over the layer16, then forming a polysilicon layer 48 over the gate oxide layer 46.The polysilicon layer can be undoped, doped in situ, or subsequentlyimplanted with a dopant, for example. Next, an insulative cap layer 50(e.g., TEOS, Si(OC₂H₅)₄, oxide, or nitride) is formed. Subsequently, thelayers 46, 48, 50 are masked, such as with patterned photoresist, andremoved from underlying active areas by etching, leaving gate stackswhich will be the transistor gates, including the transfer gate 24 andthe reset gate 28. According to the exemplary embodiment of theinvention, the photoresist patterning and etching is used to preservethe conserved areas 54, of which portions located over trench isolationregions 42 appear in FIG. 5.

In an alternative embodiment, a silicide layer 52 (shown by a dashedline in FIGS. 1 and 5-11) can be formed over the polysilicon layer 48.Additionally, a V_(t) implant can be performed during processing, as isknown in the art.

The p-well regions 22 are formed by a dopant implant 62 performed in thelayer 16. The stacks for transfer gate 24 and reset gate 28 are formedalong with the light shields of conserved sections 54. A photoresistmask 64 prevents the implant 62 from doping the area of the pixel wherethe photodiode 14 will later be formed. As an alternative, the p-typeregions 22 may be formed by a blanket implant. Of course, dopantconductivity types utilized throughout processing can easily be reversedto form a PMOS type pixel structure, as opposed to the exemplary NMOSpixel shown and described.

After forming the p-well region 22, another implant 66 is used as isknown in the art to form the n-type floating diffusion region 26adjacent the stack for transfer gate 24. N-type source/drain regions forother transistors can be formed simultaneously. The floating diffusionregion 26 acts as a source/drain region of the transfer transistor 25.Implant 66 can be performed in the implant dose range of about 1×10¹² toabout 2×10¹⁶ ions/cm². In a preferred embodiment the implant dose rangefor the implant 66 is about 4×10¹² to about 2×10¹⁵ ions/cm² and thefloating diffusion region 26 is completed by diffusion.

The layers of the photodiode 14 (i.e., layers 18, 20, 21) can be formedas shown in FIGS. 6 and 7. The exemplary photodiode 14 comprises a p-n-pstructure made of the underlying p-type layer 16, an n-type region 18within the p-type layer 16, and a p-type layer 20 above the n-typeregion 18. FIG. 6 shows the layer 16 masked with a patterned photoresist68, and another ion implantation 70 of a second conductivity type(n-type), is performed. This forms the n-type region 18 in the directlight receiving area of photodiode 14 and below the transfer gate 24. Anangled ion implantation 70 can be utilized in forming the n-type region18 to achieve certain spatial characteristics of the photodiode 14.

As shown in FIG. 7, after removing the patterned photoresist 68, aninsulating layer 72 is formed. Another mask of photoresist 74 issupplied partially over the transfer gate 24 and a dopant implant 75 iscarried out to form the top p-type layer 21 of the photodiode 14.Optionally, an angled implant for-dopant implant 75 may be utilized. Thephotodiode 14 is termed a “pinned” photodiode because the potential inthe photodiode 14 is pinned to a constant value when fully depleted.

As shown in FIG. 8, a dielectric layer 78 deposited over the pixel 12circuitry including the transfer gate 24 and reset gate 28. Thisdielectric layer 78 should be optically transparent so as not to impedelight from reaching the photodiode 14. The dielectric layer 78 cancomprise, e.g., silicon oxides or nitrides, glasses, or polymericmaterials, and can be deposited by evaporative techniques, CVD (chemicalvapor deposition), PECVD (plasma enhanced CVD), sputtering, or othertechniques known in the art.

The dielectric layer 78 may be planarized by various techniques, such asCMP or RIE etching. Advantageously, however, the expanded coverage ofthe polysilicon layer 48 resulting from preservation of the conservedsections 54 will result in a more planar deposition of dielectric layer78. Consequently, steps that were used prior to the invention forplanarizing the dielectric layer 78 can be eliminated or abbreviated.Alternatively, if a conformal dielectric layer is desired, theplanarization step is excluded.

Although sufficient light-shielding can be provided by the polysiliconlayer 48 remaining in the conserved sections 54, a further optionalupper layer light shield 80 can be formed over the dielectric layer 78by depositing a layer of opaque or nearly opaque material as a thinfilm. The upper layer light shield 80 preferably is about 100 Å to about3,000 Å thick. Upper layer light shield 80 can be made thinner thanotherwise would be the case due to the presence of the conservedsections 54 performing as light shields. The upper layer light shield 80should be of a thickness and material so that, combined with the lightabsorption of polysilicon layer 48, for example, a transmission rate ofless than 1% of impacting light is achieved. Material for the upperlevel light shield 80 can comprise, e.g., tungsten (W), tungstensilicides (WSi_(x)), poly/WSi_(x), titanium (Ti), titanium nitride(TiN), cobalt (Co), chromium (Cr), aluminum (Al), Ti/Al, TiSi₂/Al, andTi/Al/TiN, without limitation. The light shield 80 can be deposited onthe dielectric layer 78 by conventional methods, such as by evaporationtechniques, physical deposition, sputtering, CVD, etc., as either aplanar layer or a conformal layer. The light shield 80 can beelectrically conductive or electrically insulative. If formed of aconductive material, the light shield 80 can be connected to ground,thereby offering an electrical shield to isolate the underlyingcircuitry from the overlying conductive interconnect, e.g.,metallization layers, which will be formed in subsequent steps.Alternatively, the conductive light shield 80 can be used as an M1 layerto electrically connect areas of the pixel. The light shield 80 ispositioned adjacent the underlying photodiode, and comparatively muchcloser to the photodiode than in prior art light shields formed in theM1 and/or M2 metallization layers. This, combined with the lightshielding properties of conserved sections 54, also mitigates lightpiping and shadowing.

Next, as shown in FIG. 9, a patterned photoresist mask 82 is formed onthe optional light shield 80. Subsequently, the optional light shield 80is etched to form an aperture 84 over the photodiode 14. The dielectriclayer 78 can serve as an etchstop.

As shown in FIG. 10, a second dielectric layer 86 is deposited over theoptional light shield 80 and within the aperture 84 over the firstdielectric layer 78. The second dielectric layer 86 can be the same orsimilar in compositional, light transmission, and dielectric propertiesas the first dielectric layer 78 and can be deposited in a similarfashion. The second dielectric layer 86 can be planarized by CMP or RIEetching techniques, or alternatively, can be a conformal layer. Apatterned photoresist 88 is formed over the second dielectric layer 86.Subsequent etching forms opening 90, for example, through the twodielectric layers 78, 86 and the light shield 80, exposing gates andactive areas in the substrate, such as floating diffusion region 26. Theopenings provide access for making interconnections between elements ofthe pixel circuitry, to a supply voltage, and to output and controlcircuits.

Conductors for active circuit areas of the pixel 12, for example, areformed within openings such as opening 90, shown in FIG. 11. Optionally,a thin insulating layer (not shown) can be deposited within the openings90 to electrically isolate the optional light shield 80, if conductive,from the conductors. One such conductor 92 is shown to connect with thefloating diffusion region 26. Over the second dielectric layer 86 and inelectrical communication with the conductor 92, an interconnect layer94, preferably of metal, is deposited to form an M2 layer. Generally,the conductive interconnect layer 94 should not extend over the aperture90, or photodiode 14, for example, particularly if the conductiveinterconnect layer is composed of an opaque or translucent material.However, transparent or semi-transparent materials, e.g., polysilicon,can be used for the conductive interconnect layer 94, and can overliethe photodiode 14, if desired.

The floating diffusion region 26 is electrically connected with thesource follower gate 32 through standard metallization steps, e.g., aconductor 36 connects to the floating diffusion region 26 and the secondconductor 38 (see FIG. 3) connects to the source follower gate 32,between which is formed a conductive interconnect layer 94. Conductor 36is in electrical communication with the M2 conductive interconnect layer94 and thereby with the source follower gate 32 and the rest of theintegrated circuit, of which the photodiode 14 is a part for supplyingtransduced charge to the pixel switching circuitry. Additionalprocessing can follow, such as formation of an overlying dielectriclayer 96 and a third conductive interconnect layer 98 (M3), as known inthe art. It should be understood that FIGS. 10 and 11 only show oneconductive via as exemplary. Other vias as needed for operation of thepixel are similarly formed.

By retaining the conserved sections 54 as light shields, and optionallyproviding upper light shields 80, the third conductive metalinterconnect layer 98 (M3) for the pixel array can be made optional.Conductive metal interconnect layer 98 was one layer previously employedfor light shielding. However, the M3 layer is optional. Eliminating theM3 layer reduces device size, complexity and cost, and reduces thenumber of necessary processing steps.

Additional features of the pixel structure fabricated after the FIG. 11steps include passivation layers 100, 102, color filter array 104,polyimide layer 106, and lens array 108, for example, as shown in FIG.1.

The polysilicon layer 48 used to form gate structures in the embodimentsdiscussed above may also sometimes be used to provide buried pixelinterconnect structures. In such case the conserved sections areportions of the polysilicon layer 48 not used for gates orinterconnects.

FIG. 12 illustrates a block diagram for a CMOS imaging device 110 havinga pixel array 112 incorporating pixels 12, 13, etc., constructed in themanner discussed above in relation to FIGS. 1-11. Pixel array 112features a plurality of pixels arranged in columns and rows. The pixelsof each row in pixel array 112 can all be turned on at the same time bya row select line and the pixels of each column are selectively outputby a column select line. A plurality of row and column lines is providedfor the entire pixel array 112. The row lines are selectively activatedby a row driver 114 in response to a row address decoder 116 and thecolumn select lines are selectively activated by a column driver 120 inresponse to a column address decoder 122. Thus, a row and column addressis provided for each pixel.

The CMOS imaging device 110 is operated by a control circuit 124 whichcontrols the address decoders 116, 122 for selecting the appropriate rowand column lines for pixel readout, and the row and column drivercircuits 114, 120 which apply driving voltage to the drive transistorsof the selected row and column lines. A memory 126, e.g., a FLASH memoryor an SRAM, can be in communication with the pixel array 112 and controlcircuit 124. A serializer module 128 and SFR (Special Function Register)device 130 can each be in communication with the control circuit 124.Optionally, a localized power source 132 can be incorporated into theimaging device 110.

Typically, the signal flow in the imaging device 110 would begin at thepixel array 112 upon its receiving photo-input and generating a charge.The signal is output to a read-out circuit and then to ananalog-to-digital conversion device. The digitized signal is transferredto a processor, then the serializer, and the serialized signal can beoutput from the imaging device to external hardware.

FIG. 13 shows system 200, a typical processor based system, whichincludes an imaging device 110 illustrated in FIG. 12 as an input deviceto the system 200. The imaging device 110 may also receive control orother data from system 200 as well. Examples of processor based systems,which may employ the imaging device 110, include, without limitation,computer systems, camera systems, scanners, machine vision systems,vehicle navigation systems, video telephones, surveillance systems, autofocus systems, star tracker systems, motion detection systems, imagestabilization systems, and others.

System 200 includes a central processing unit (CPU) 202 thatcommunicates with various devices over a bus 204. Some of the devicesconnected to the bus 204 provide communication into and out of thesystem 200, illustratively including an input/output (I/O) device 206and imaging device 110. Other devices connected to the bus 204 providememory, illustratively including a random access memory (RAM) 210, ahard drive 212, and one or more removable memory devices, such as afloppy disk drive 214, compact disk (CD) or digital video disk (DVD)drives 216, flash memory cards, etc. The imaging device 110 may becombined with a processor, such as a CPU, digital signal processor, ormicroprocessor, in a single integrated circuit.

The processes and devices described above illustrate exemplary methodsand devices out of many that could be used and produced according to thepresent invention. The above description and drawings illustrateexemplary embodiments which achieve the objects, features, andadvantages of the present invention. It is not intended, however, thatthe present invention be strictly limited to the above-described andillustrated embodiments. For example, the invention is not limited tothe use of a gate polysilicon or oxide layer as a light shield. Asnoted, the conserved sections of these layers may also be used to gatherlight.

Referring now to FIG. 14, substrate 300 and fabrication layer 301 (showngenerically) support polysilicon layer 302 used for a top electrodeplate 304 of a capacitor 306, for example. Polysilicon layer 302 alsocould be patterned to produce conserved sections used as light shieldsor for light gathering, in addition to, or separate from, thepolysilicon gate layer conserved sections. Accordingly, modifications,though presently unforeseeable, of the present invention that comewithin the spirit and scope of the following claims should be consideredpart of the present invention.

1. An imager pixel comprising: a substrate; a photoactive region havingan admitting face and supported by the substrate, and arranged totransduce light incident the admitting face; pixel circuit structure ina first section of a fabrication layer supported by the substrate; and alight-shielding region in a second section of the fabrication layerarranged adjacent and outside the photoactive region and configured toabsorb light incident the imager pixel outside the photoactive regionand the first section. 2-49. (canceled)